Chemistry Reference
In-Depth Information
Concept Keys
The thermodynamic stability of a complex can be expressed in terms of a stability
constant, which reports the ratio of complexed ligand to free metal and ligand in an
equilibrium situation.
The stability of metal complexes is influenced by an array of effects associated with
metal or ligand: the size of and charge on the metal ion, hard-soft character of
metal and ligand, crystal field effects, base strength of the ligand, ligand chela-
tion where it is possible, chelate ring size where applicable, and ligand shape
influences.
For complexation of a set of ligands, each successive addition has a stability constant
(
K
n
) associated with it, with each successive stability constant progressively smaller
for a fixed stereochemistry. An overall stability constant (
n
=
K
1
·
K
2
·
...
·
K
n
)ex-
presses the stability of the completed assembly.
Whereas thermodynamic stability is concerned with complex stability at equilibrium,
kinetic stability is concerned with the rate of formation of a complex leading to equi-
librium. Complexes undergoing reactions rapidly are labile, those reacting slowly
are inert.
Reactions of complexes may involve changes to the coordination sphere, the metal
oxidation state and/or the ligands themselves. Partial or complete ligand replacement
(substitution) is the most common reaction met in metal complexes.
Octahedral substitution reactions may occur through a dissociative mechanism fea-
turing a five-coordinate transition state (ligand loss rate-determining), an associa-
tive mechanism featuring a seven-coordinate intermediate (ligand addition rate-
determining), or else by an interchange mechanism where ligand exchange happens
in a concerted manner. Similar concepts can be applied to other coordination num-
bers and shapes.
Mechanisms for optical and geometrical isomerization reactions similar to those em-
ployed for substitution reactions can be envisaged. Additionally possible is a twist
mechanism, involving distortion of the polyhedral framework in the activated state
but in which no ligands depart or join the coordination sphere.
Oxidation-reduction (or electron transfer) reactions involving two metal complexes
may occur by one of two mechanisms: outer sphere (no direct involvement of the
coordination sphere) or inner sphere (where one ligand on one complex forms a
bridge to the other metal in the transition state).
Electron addition or extraction from a complex to cause reduction or oxidation may
also be achieved electrochemically (at an electrode in an electrochemical cell) or
radiolytically (using an aquated electron or hydroxyl radical).
Further Reading
Atwood, J.D. (1997)
Inorganic and Organometallic Reaction Mechanisms
, 2nd edn, Wiley-VCH
Verlag GmbH, Weinheim, Germany. Classical examples are covered clearly for the student in
good depth; reactions, procedures for their examination, and mechanisms by which they react are
explained - with the bonus of coordination complexes and organometallic systems presented in a
single text.
